Transparent Energy-Harvesting Devices

ABSTRACT

An energy harvesting system is provided. The energy harvesting system includes a waveguide, a luminophore embedded in the waveguide, and a solar photovoltaic array or a solar photovoltaic cell coupled to the waveguide. The energy harvesting system is visibly transparent, having an average visible transmittance of greater than about 50% and a color rendering index of greater than about 80 at normal incidence to the waveguide.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation of U.S. patent application Ser. No.16/697,762, filed on Nov. 27, 2019, which is a continuation of U.S.patent application Ser. No. 14/220,850, filed on Mar. 20, 2014, whichclaims the benefit of U.S. Provisional Application No. 61/947,187, filedon Mar. 3, 2014, and U.S. Provisional Application No. 61/804,053, filedon Mar. 21, 2013. The entire disclosures of each of the aboveapplications are incorporated herein by reference.

BACKGROUND

The present application generally pertains to solar concentrators andmore particularly to transparent solar concentrators for integratedsolar windows.

Integrating solar-harvesting systems into the built environment is atransformative route to capture large areas of solar energy, lowereffective solar cell installation costs, and improve buildingefficiency. Widespread adoption of solar-harvesting systems in abuilding envelope, however, is severely hampered by difficultiesassociated with mounting traditional solar modules on and aroundbuildings due to cost, architectural impedance, and most importantly,aesthetics.

The concept of luminescent solar concentrators (“LSCs”) is well known,and with recent advances in phosphorescent and fluorescent luminophoreefficiencies, LSC system efficiencies have increased to 7.1%. Althoughoptical funneling of light limits the overall system conversionefficiency to less than ten percent (without LSC stacking), it candramatically reduce the area of expensive solar cells needed, drivingdown the overall installed cost and increasing the ratio of electricitygeneration to solar cell surface area. Because of the high cost of glassand real-estate that factor into the module and the balance of systemscosts, respectively, such LSCs have rarely been adopted in solar-farmpractice despite the increasing performance and potential for low modulecosts. Furthermore, there has been demonstrated interest in utilizingLSCs as architectural windows. To date, however, these systems have beenlimited to absorption and emission (glow) in the visible part ofspectrum, hindering widespread adoption of such devices in windows. Ingeneral, the purpose of windows is to provide natural lighting with aview; that is, most people prefer not to work behind strongly coloredglass. A high level of untinted-transparency is therefore desirable forubiquitous adoption.

The performance of LSCs can be understood by the component efficiencies:luminophore photoluminescence efficiency (quantum yield), solar spectrumabsorption efficiency, waveguide (trapping) efficiency, solar cellefficiency, and transport (re-absorption) efficiency. The highestperformance LSCs utilize phosphorescent organic molecules or blends ofmultiple fluorophores (such as quantum dots or organic dyes) that act toreduce reabsorption (Stokes shift) losses and enhance overall absorptionefficiencies across the spectrum. The highest efficiencies reported(6-7%) have been for relatively small plates (<0.1 m²), since largerLSCs sizes suffer substantial reabsorption losses that limitefficiencies to <5%.

It has long been recognized that LSCs are most limited by reabsorptionlosses, particularly for larger plate sizes. Indeed, much of theresearch with LSCs has focused on the reduction of these reabsorptionlosses through increasing Stokes shifts with organic phosphors, multipledye optimization to artificially increase the Stokes shift or resonanceshifting, applicable only to neat-film dye layers less than severalmicrons thick.

Previous efforts to construct transparent solar-harvesting architectureshave focused on: (1) semi-transparent thin-film photovoltaics thattypically have severe tinting or limited transmission or have aninherent tradeoff between efficiency and transparency, (2) LSCsincorporating colored chromophores that absorb or emit in the visible,or (3) optical systems using wavelength dependent optics that collectdirect light only and require solar tracking. All of these approachesare severely limited in their potential for window applications due toaesthetic properties, bulkiness, or considerably limited transparency.These approaches suffer from an inherent tradeoff between powerconversion efficiency (“PCE”) and visible transparency (“VT”), sinceboth parameters cannot be simultaneously optimized in conventionaldevices. Architectural adoption is impeded further with typical organicphotovoltaics (PVs) that have peaked absorption within the visiblespectrum, resulting in poor color rendering index (“CRI”), high coloredtinting, and poor natural lighting quality. In contrast, it would bedesirable to obtain visibly transparent, UV/NIR-selective solarconcentrators to avoid aesthetic tradeoffs (low VT or CRI) that hinderarchitectural adoption and provide a clear route to large area scaling.

FIG. 1 shows an illustration of a cross section of a luminescent solarconcentrator (LSC) 100 in a traditional configuration. The SC 100comprises a first inorganic solar cell 105 and a second inorganic solarcell or a reflective film 110, a substrate 115, and a waveguideredirecting material 120. The waveguide redirecting material 120 can bean NIR luminescent dye or scattering particles. Long bracket 125represents incoming solar flux, short bracket 130 represents NIR light,and block arrow 135 represents visible light. Arrows 140 representwaveguided NIR light. The factors that negatively affect the efficiencyof the LSC 100 include poor luminophore photoluminescence efficiency(quantum yield), poor solar spectrum absorption efficiency, poorwaveguide (trapping) efficiency, poor solar cell efficiency, and poortransport (re-absorption) efficiency.

Various conventional devices employ a luminescent solar collector havingluminescent or scattering agents dispersed throughout. Exemplary U.S.Patent Nos. include: U.S. Pat. No. 4,155,371 entitled “Luminescent SolarCollector” which issued to Wohlmut et al. on May 22, 1979; U.S. Pat. No.4,159,212 entitled “Luminescent Solar Collector” which issued to Yerkeson Jun. 26, 1979; U.S. Pat. No. 4,357,486 entitled “Luminescent SolarCollector” which issued to Blieden et al. on Nov. 2, 1982; 2009/0027872entitled “Luminescent Object Comprising Aligned Polymers having aSpecific Pretilt Angle” which published to Debije et al. on Jan. 29,2009; and 2010/0288352 entitled “Integrated Solar Cell Nanoarray Layersand Light Concentrating Device” which published to Ji et al. on Nov. 18,2010. All of these references are incorporated by reference herein.

SUMMARY

In accordance with the present technology, a transparent luminescentsolar concentrator (TLSC) is provided. Systems include those comprisinga solar photovoltaic array, a waveguide, and a waveguide redirectingmaterial. In various embodiments, the waveguide redirecting material iscoupled to the solar photovoltaic array. The waveguide redirectingmaterial may be embedded in the solar photovoltaic array. In someembodiments, the waveguide redirecting material is juxtaposed to the atleast one solar photovoltaic array. The waveguide redirecting materialmay comprise a scattering nanoparticle. The system may comprise aplurality of arrays.

In one aspect, a luminescent solar concentrator (LSC) selectivelyabsorbs NIR light with wavelengths of greater than about 650 nm. Afurther aspect of a luminescent solar concentrator employs scatteringnanocrystal clusters embedded in a polymeric matrix.

The present apparatus and method are advantageous over traditionaldevices. For example, the present luminescent solar concentrator ishighly transparent to human vision, thereby making it ideally suited foruse in viewing windows in a structural building, greenhouses, automotivevehicle, aircraft, watercraft, or the like. Moreover, the luminophoreand host composition aspect of the present apparatus achieve a quantumyield greater than about 10-25%.

To overcome prior disadvantages, the present low-cost, solarconcentrator is highly transparent in the visible spectrum, and enablesintegration onto window and glazing systems in the building envelope.The excitonic character and structured absorption of molecular dyes willproduce near-infrared (“NIR”) LSC architectures that selectively harvestNIR light by waveguiding deeper-NIR luminophore emission to highefficiency segmented solar cell arrays that are embedded within thewaveguide, thereby reducing any visual impact and minimizing the amountof expensive solar material required while extending the photonharvesting range into the NIR. Specific luminophores that can harnessthe majority of near-infrared photons are ideally suited for thesetransparent systems. Luminophore blends of cyanine, cyanine salts, andsquaryliums are particularly suited for TLSCs with high transparency andminimal tinting. Accordingly, this apparatus optimizes transparent solarconcentrators with system efficiencies greater than about 0.1-5%,average visible transmittance of greater than about 70%, and a colorrendering index of greater than about 85 for widespread windowdeployment. These SCs generate electricity from both direct and incidentlight, and enhance the low-light efficiency through solar concentrationwhile maintaining window glass aesthetics and architectural designfreedom.

DRAWINGS

The drawings described herein are for illustrative purposes only ofselected embodiments and not all possible implementations, and are notintended to limit the scope of the present disclosure.

FIG. 1 is an illustration of a traditional luminescent or scatteringsolar concentrator;

FIG. 2 is an illustration showing solar flux around a building inBoston, Mass.;

FIGS. 3A-3D show illustrations of a first solar concentrator;

FIGS. 4A-4D show illustrations of a second solar concentrator;

FIG. 5 is an illustration of solar concentrators with reflectivemirrors;

FIGS. 6A-6J are graphs showing absorption and emission spectra ofvarious luminophores;

FIGS. 7A-7C show (7A) a schematic of a transparent luminescent solarconcentrator; (7B) CY and HITC structures, and (7C) a photograph of atransparent LSC system incorporating CY dye highlighting the need forboth absorption and emission outside the visible spectrum;

FIGS. 8A-8B show (8A) normalized absorption (circles) and emission(squares) spectra of the NIR absorbing luminophores CY and HITCI, and(8B) shows the measured quantum yield (QY) and absorption of CY andHITCI in dichloromethane as a function of concentration; solid lines arefits to Equation 1 for the QY and the Beer-Lambert law for absorption;

FIGS. 9A-9D show (9A) light intensity dependence of the Voc, FF, powerefficiency, and responsivity of the CY device, (9B) current density as afunction of voltage for the fully assembled LSC systems with twoluminophores, (9C) EQE of the LSC system as a function of wavelengthmeasured from 0.02 m to 0.07 m, at 10 mm increments, and (9D) calculatedEQE as a function of CY LSC length with the measured EQE shown ascircles; and

FIG. 10 shows a calculation of CY LSC optical efficiency as a functionof LSC length with as a function of simulated Stokes shift, wherein CYand HITCI are references with Stokes shifts of 20 nm and 30 nm,respectively.

Corresponding reference numerals indicate corresponding parts throughoutthe several views of the drawings.

DETAILED DESCRIPTION

The following description of technology is merely exemplary in nature ofthe subject matter, manufacture and use of one or more inventions, andis not intended to limit the scope, application, or uses of any specificinvention claimed in this application or in such other applications asmay be filed claiming priority to this application, or patents issuingtherefrom.

The description and specific examples, while indicating embodiments ofthe technology, are intended for purposes of illustration only and arenot intended to limit the scope of the technology. Moreover, recitationof multiple embodiments having stated features is not intended toexclude other embodiments having additional features, or otherembodiments incorporating different combinations of the stated features.Specific examples are provided for illustrative purposes of how to makeand use the compositions and methods of this technology and, unlessexplicitly stated otherwise, are not intended to be a representationthat given embodiments of this technology have, or have not, been madeor tested.

Embodiments of the present apparatus and method are directed to animproved light harvesting system that incorporates a segmented solararray at a top and/or bottom surface (rather than an edge of awaveguide). The system, which comprises a luminescent solar concentrator(LSC), can selectively harvest light with a wavelength in anear-infrared (NIR) region of an electromagnetic spectrum. Accordingly,by selectively harvesting NIR light, the luminescent solar concentratorcan be transparent, or substantially transparent, to a human eye.

The present LSC can be used in creating three-dimensional structures,such as solar towers, obelisks, and windows to enhance solar collection.These structures can collect substantially more flux than solar trackingunits of an equivalent footprint. Likewise, buildings can act asexcellent solar collectors, collecting sunlight effectively throughoutthe day on each face. For example, in FIG. 2 , it is estimated that thetotal solar flux density from all four sides of a vertical building inBoston (9.3 kW-hr/vertical-m²-day) is substantially more than for asolar tracking unit, even, of equivalent footprint (6.0 kW-hr/horizontal-m²-day), and even greater if the total vertical area utilized in thebuilding is accounted for; while South-facing vertical windows will givethe highest solar flux (and therefore power output), East-West facingwindows extend the useful power production throughout the day.

Roughly 17% of all building electricity in the U.S. is used forlighting. Incentivizing solar window adoption can encourage architectsto utilize more window space, increasing natural lighting, and reducinglighting costs. Reproducing the solar spectrum is the goal of mostartificial lighting applications. Each solar window will impart a smallmodification to the spectrum transmitted. Thus, the color renderingindex (“CRI”) is used for evaluation by convoluting the transmissionspectra with the solar spectrum (e.g., AM1.5G, etc.). The colorrendering framework is useful for defining the visible part of thespectrum that should be transmitted and the components that can beutilized for power generation. For example, the 1931 CIE coordinateplots end at approximately 660 nm and 440 nm, with very little photopicresponse above and below these limits, respectively, defining the activespectral range to utilize as ≤440 nm and ≥660 nm. While there is someresponse in the ranges from 380-440 nm and 660-720 nm, the response isparticularly limited (<2% of the peak response) so that thesewavelengths do not contribute significantly to the overall perception ofcolor.

As used herein, the CRI is the range of perceptible visible light. TheCRI is subsequently utilized to determine thermodynamic efficiencylimits for visibly transparent solar cells. Specifically, the CRI is aquantitative metric for evaluating the quality of lighting systems andcan be utilized to evaluate the level or perceptible color-tinting of awindow. CRIs are calculated based on ideal transmission profiles(step-functions) in combination with International Commission onIllumination (CIE) 1976 three-dimensional uniform color space (CIELUV),CIE 1974 test-color samples, and with correction for chromaticadaptation (non-planckian-locus), when necessary, according to:

${CRI} = {\frac{1}{8}{\sum\limits_{i = 1}^{8}\left( {100 - {4.6\sqrt{\left( {\Delta L_{i}^{*}} \right)^{2} + \left( {\Delta u_{i}^{*}} \right)^{2} + \left( {\Delta v_{i}^{*}} \right)^{2}}}} \right)}}$

where ΔL_(i)*, Δu_(i)*, and Δv_(i)* are the difference in lightness (L*)and chromaticity coordinates (u*, v*) between each color sample, i (8 intotal) “illuminated” with a fixed reference solar spectrum (AM1.5G) andthe transmission sources (T(λ)·AM1.5(λ)). The weighted average visibletransmittance (VT) is calculated according to:

${VT} = \frac{\int{{T(\lambda)}{P(\lambda)}{S(\lambda)}d\lambda}}{\int{{P(\lambda)}{S(\lambda)}d\lambda}}$

where λ is the wavelength, T is the transmission spectrum, P is thephotopic response of the human eye, S is the solar energy flux, and theintegration is preformed over a sufficient wavelength range tocompletely encompass P (e.g., 300-900 nm) so that it is accordinglyindependent of any defined visible wavelength range. CRI and average VTare described in detail in Lunt, “Theoretical Limits for VisiblyTransparent Photovoltaics.” Appl. Phys. Lett., 101, 043902 (2012), whichis incorporated herein by reference in its entirety.

A significant fraction (˜15%) of building electricity is utilized forair-conditioning. Conversion of electricity to heat is highly efficient(nearly 100%), while cooling efficiencies are significantly lower. Thepresent LSCs, used as solar windows, can dramatically reduce unwantedsolar heating from infrared flux by utilizing this energy forelectricity generation and rejecting the rest from transparent,NIR-selective mirrors. This added benefit of utilizing this unwantedresource for electricity generation could compliment or replace currentlow solar heat gain coefficient (SHGC) coating technologies. Thisheat-rejection functionally can enhance the effective LEC from directpower generation by 50-100%.

A great fraction of the solar photon flux is in the NIR (˜74%) region ofthe electromagnetic spectrum. This limitation is overcome by positioninghighly-segmented, 50-100 μm wide, solar cell arrays commerciallyavailable and: (1) wired in parallel or in series, or (2)laser-patterned as meshes and embedded throughout the LSC waveguide,allowing for the use of smaller Stokes shift NIR fluorophores.

With reference to FIG. 3 , a first embodiment of a solar concentrator(SC) 300, shown in a first orientation FIG. 3A and a second orientationFIG. 3B, includes a segmented inorganic solar photovoltaic array 305positioned at the top, middle, or bottom of a waveguide 310. In variousembodiments, the solar array 305 is a mesh comprising wires, microwires,nanowires, ribbons, slivers, spheres, dots, or combinations thereof, orthe like arranged within or placed on a surface of the waveguide. Morethan one photovoltaic array 305 can be utilized to reduce thermallosses. Cost and availability, however, will ultimately drive the celldesign. In some embodiments, the SC 300 comprises a solar cell on anedge of the waveguide 310. As a non-limiting example, GaAs cells couldboost solar cell efficiency (η_(SC)) just over 30% compared to Si, butestimated costs for these cells are more than double or triple that ofSi. For the least expensive 15-20% Si modules available, themonochromatic efficiency translates η_(PV)=22-24% for NIR LSC (emission900 nm) material sets. Accordingly, it is envisioned that LSC systemscan be efficiently coupled with Si as less expensive higher bandgapcells (e.g., GaAs, and GaInP) become available. The solar array 305 orsolar cell can comprise any material known in the art. Non-limitingexamples of solar array and solar cell materials include germanium (Ge);amorphous germanium (a-Ge); gallium (Ga); gallium arsenide (GaAs);silicon (Si); amorphous silicon (a-Si); silicon-germanium (SiGe);amorphous silicon-germanium (a-SiGe); gallium indium phosphide (GaInP);copper indium selenide, copper indium sulfide, or combinations thereof(CIS); copper indium gallium selenide, copper indium gallium sulfide, orcombinations thereof (CIGS); cadmium telluride (CdTe); perovskites (PV),such as CH₃NH₃PbI₃, CH₃NH₃PbCl₃ and CH₃NH₃PbBr₃; and combinationsthereof.

Referring again to FIG. 3 , the waveguide 310 comprises a transparent orsubstantially transparent substrate 315, such as, glass, plastic,poly(methyl methacrylate) (PMMA), poly-(ethylmethacrylate) (PEMA), or(poly)-butyl methacrylate-co-methyl methacrylate (PBMMA) having awaveguide redirecting material 320 formed therewith, such as throughcoating or spraying on the substrate 315 or layering dyed sheets asstrata between alternating stacks of the substrate 315. In someembodiments, the waveguide redirecting material 320 is embedded with thesubstrate 315. The waveguide redirecting material 320 can be avisibly-transparent NIR luminescent dye or scattering particles. Theluminescent dye or scattering particles can be nanocrystal clustersembedded in a matrix. Because glass absorption contributes to loss,highly transparent low iron glasses, such as Diamont® low iron glass,Planilux® transparent glass from Saint Gobain (Paris, France), andBorofloat® 33 borosilicate glass from Schott (Mainz, Germany), arepreferred. These glasses are substrates of choice for commercialdeployment over other polymer bases waveguides, which are rarelyutilized as the base component in windows and exhibit particularly lowabsorption coefficients comparable to that of PMMA. Advantageously, theproximity of the solar cell to the dyes allows a smaller Stokes shiftdye to be used, while still reducing thermal losses and improvingoverall system efficiency.

As shown in FIG. 3C, in various embodiments, the solar array 305 can beembedded within the substrate 315. The solar cell 305 can be embedded ina first position 325, near a first surface 340 of the SC 300.Alternatively, the solar array 305 can be embedded in a second position330, in a middle area of the SC 300. Also, the solar array 305 can beembedded in a third position 335, near a second surface 345 of the SC300. The middle area 300 is between the first surface 340 and the secondsurface 345. Additionally, the SC 300 can comprise a plurality of solarphotovoltaic arrays 305, positioned at different locations within thesubstrate 315. The solar array 305 can also be at or near one or both ofthe first surface 340 or the second surface 345. In one embodiment, theSC 300 comprises a solar cell at a side of the waveguide 310 that isadjacent to the first surface 240 and the second surface 345. In yetanother embodiment, shown in FIG. 3D, a solar cell array 350 has athickness 355 that is about the same overall thickness 360 of the SC300.

As exemplified in FIG. 4 , a second embodiment of a solar concentrator400, substantially similar to that described with respect to the firstembodiment (FIG. 3 ), is shown in a first orientation in FIG. 4A and asecond orientation in FIG. 4B. However, a solar photovoltaic array 405is wired in series may include adducts 410, such as spheres or dots. Forexample, the solar array 405 may comprise a Si sphere solar arrayconnected by thin-wire electrical connections. In a preferredembodiment, the solar array 405 comprises spheres that are wiredtogether to form a mesh. Advantageously, using spherical or multi-sidedsolar arrays 405 further increases system efficiency as each cell cancapture both direct and indirect sunlight at multiple angles. As shownin FIGS. 4C and 4D, the solar array 405 can be located at differentpositions within the substrate 315, at or near the surfaces of the SC400, or the solar array 405 can have a thickness 455 that is about thesame overall thickness 460 of the SC 400. In some embodiments, the SC400 can comprise a plurality of solar photovoltaic arrays cells 405,located at different positions within the substrate 315. In otherembodiments, the SC 400 comprises a solar cell positioned on an edge ofthe waveguide 310 that is adjacent to the surfaces. The solar array 405or solar cell can be made of any material known in the art, as discussedabove.

By positioning segmented solar cell arrays at the top or bottom of thewaveguide, it is possible to capture luminesced light before it is lostoptically due to dye reabsorption. Alternatively, the solar arrays canbe embedded within the waveguide. This loss of reabsorption isparticularly beneficial for fluorescent materials with small Stokesshifts. Depending on where the solar cell arrays are positioned in thewaveguide, the waveguide redirecting material can be either embedded in,or juxtaposed to, the solar photovoltaic array. Using dyes with smallStokes shifts as a waveguide redirecting material enables a use of solarcells with larger open circuit voltage, V_(oc). Such dyes reduce thermallosses and improve overall system efficiency. Also, by positioning thearrays at the top or bottom of the waveguide, a fraction of forwardemitted light can be captured, which further reduces optical losses, andit enhances the efficiency of spatially segmented solar cells byutilizing a greater fraction of NIR light between cells. The positioninggenerates an architecture that is simpler than thin-film transparentphotovoltaics for transparent power producing windows. Furthermore, thearchitecture can lead to increased window lifetime as well.

As shown in FIG. 5 , a solar concentrator (SC) 500 can comprise a firstwavelength-dependent mirror 505. The first wavelength-dependent mirror505 can have a reflectivity of NIR light corresponding to only anemission spectrum of a waveguide redirecting material as shown in graph550. Therefore, the first wavelength-dependent mirror 505 is transparentto visible light 510, but reflects NIR light 515 in an emission rangeshown in graph 550. The first wavelength-dependent mirror 505 can befunctionally coupled to a first surface 520 of the SC 500, whichcomprises a waveguide redirecting material, such as a luminophore.Alternatively, the first wavelength-dependent mirror 505 can befunctionally coupled to a second surface 525 of the SC 500, or to bothsurfaces 520,525. In other embodiments, the waveguide redirectingmaterial is on the second surface 525 or embedded within the waveguide.

With further reference to FIG. 5 , the SC 500 can comprise a secondwavelength-dependent mirror 530. The second wavelength-dependent mirror530 can have a reflectivity of NIR light corresponding to bothabsorption and emission spectra of a waveguide redirecting material asshown in graph 560. The second wavelength-dependent mirror 530 istransparent to visible light 510, but reflects NIR light 515 in anemission range shown in graph 560. The second wavelength-dependentmirror can be functionally coupled to the first surface 520 of the SC500, to the second surface 525 of the SC 500, or to both surfaces520,525. In a preferred embodiment, the first wavelength-dependentmirror is functionally coupled to the first surface 520 of the SC 500,and the second wavelength-dependent mirror is functionally coupled tothe second surface 525 of the SC 500. As described above, a photovoltaicarray can be positioned on the first surface 520, the second surface525, or within the waveguide. In various embodiments, the SC 500comprises a plurality of solar arrays positioned at any of the firstsurface 520, the second surface 525, or within the waveguide. In yetother embodiments, the SC 500 comprises a solar positioned at an edge ofthe SC 500 that is adjacent to the first and second surfaces 520, 525.

Incorporation of visibly transparent, selective NIR mirrors 505, 530 intransparent photovoltaics (TPVs) substantially improves power conversionefficiencies by 50-100%. Similarly, the incorporation of these mirrorsimproves the optical efficiency at low plate dimension by greater thanabout 20% while reducing the quantity of dye needed by half for a givenoptical density. For LSC sizes of greater than about 0.5 m², thesemirrors are helpful in mitigating any surface and bulk scatteringimperfections that could reduce system efficiencies. The mirrors can becoatings that improve collector absorption and increase waveguiding.Moreover, these coating layers are very similar to low-e-coatings thatare already ubiquitously deployed and can complement or replace much oftheir functionality for heat rejection. Alternating layer combinationsof TiO₂, SiO₂, and Al₂O₃ can be grown by e-beam evaporation, pulsedlaser deposition, plasma-enhanced sputtering, thermal deposition,chemical vapor deposition, or solution deposition to optimize overallcolor impact and performance.

The scattering particles can be metal oxide clusters, metalnanoclusters, or both. The metal oxide clusters comprise metal oxides.Non-limiting examples of metal oxides include TiO₂, SiO₂, ZnO and Al₂O₃.Metal nanoclusters comprise metals that are suitable for scattering NIRlight. Non-limiting examples of metals suitable for scattering NIR lightinclude Ag, Au, Al, Cu, and Pt. The scattering particles can have auniform size, or the size can vary to result in a particle sizedistribution.

The fluorescent or phosphorescent dyes comprise luminophores, which actas waveguide redirecting materials. As used herein, “luminophores”refers to inorganic or organic compounds that manifest luminescence,wherein the luminescence is fluorescence or phosphorescence.Luminophores that harness light in the NIR region of the electromagneticspectrum are ideally suited for the present transparent systems.Individual luminophore performance is improved through combinations ofchemical, purification, architecture, host-guest interactions and photonmanagement with transparent NIR mirror design, and fabrication.Preferably, the luminophores absorb NIR light of wavelengths greaterthan about 650 nm. 650 nm is the wavelength of light at which human'sbegin to loose sensitivity to color perception (overall colorrendering), as demonstrated by Lunt, “Theoretical Limits for VisiblyTransparent Photovoltaics.” Appl. Phys. Lett., 101, 043902, 2012, whichis incorporated herein by reference in its entirety. More preferably,the luminophores absorb NIR light of wavelengths greater than about 700nm, or wavelengths greater than about 800 nm. In various embodiments,the waveguide redirecting material is a visibly-transparent,NIR-absorbing, and NIR-emitting luminophore.

Cyanine dyes, cyanine salts, and naphthalocyanine derivatives arepreferred NIR fluorophores. Derivatives of well-known dyes historicallyutilized in dye-lasers that have shown the highest relative stabilityinclude naphthalocyanine derivatives (i.e., analogous to the wavelengthselective molecules previously exploited in transparent solar cells) andthiacarbocyanine salts with variable anion substitutability.

Thiacarbocyanine can be tailored through their alkene length (di, tri,tetra, etc.) to vary the electronic band gap (emission range). For agiven bandgap, the quantum yield of these materials can be directlytailored through anion modification (Cl⁻, I⁻, ClO₄ ⁻, etc.).Extended-conjugation molecule derivatives of phthalocyanines,phorphyrins, naphthalocyanines, carbon nanotubes, and small-gap polymerscan be employed to cover a significant portion of the NIR spectrum withhigh quantum yields.

Preferred luminophores include 3,3′-diethtylthiacarbocyanine iodide(DTCI); 2-[7-(1,3-dihydro-1,3,3-trimethyl-2H-indol-2-ylidene)-1,3,5heptatrienyl]-1,3,3-trimethyl-3H-indolium iodide (HITCI);2-[2-[3-[[1,3-dihydro-1,1dimethyl-3-(3-sulfopropyl)-2H-benz[e]indol-2-ylidene]-2-[4-(ethoxycarbonyl)-1piperazinyl]-1-cyclopenten-1-yl]ethenyl]-1,1-dimethyl-3-(3-sulforpropyl)-1Hbenz[e]indolium hydroxide, inner salt, compound withn,n-diethylethanamine(1:1) (IR144);2-[2-[2-Chloro-3-[(1,3-dihydro-3,3-dimethyl-1-propyl-2H-indol-2ylidene)ethylidene]-1-cyclohexen-1-yl]ethenyl]-3,3-dimethyl-1-propylindoliumiodide (IR780); aluminum 2,3-naphthalocyanine chloride (AlNc); silicon2,3-naphthalocyanine dichloride (SiNc);2-[2-[3-[(1,3-Dihydro-3,3-dimethyl-1-propyl-2H-indol-2-ylidene)ethylidene]-2-(phenylthio)-1-cyclohexen-1-yl]ethenyl]-3,3-dimethyl-1-propylindoliumperchlorate (IR792);5,5′-Dichloro-11-diphenylamino-3,3′-diethyl-10,12-ethylenethiatricarbocyanineperchlorate (IR140); zinc2,11,20,29-tetra-tert-butyl-2,3-naphthalocyanine (ZnNc);3-(6-(2,5-dioxopyrrolidin-1-yloxy)-6-oxohexyl)-1,1-dimethyl-2-((E)-2-((E)-3-((E)-2-(1,1,3-trimethyl-1H-benzo[e]indol-2(3H)-ylidene)ethylidene)cyclohex-1-enyl)vinyl)-1H-benzo[e]indoliumchloride (Cy7.5 NHS ester);1-(6-((2,5-dioxopyrrolidin-1-yl)oxy)-6-oxohexyl)-3,3-dimethyl-2-((E)-2-((E)-3-((E)-2-(1,3,3-trimethyl-5-sulfonatoindolin-2-ylidene)ethylidene)cyclohex-1-en-1-yl)vinyl)-3H-indol-1-ium-5-sulfonate(sulfo-Cy7 NHS ester);1-(6-(2,5-dioxopyrrolidin-1-yloxy)-6-oxohexyl)-3,3-dimethyl-2-((E)-2-((E)-3-((E)-2-(1,3,3-trimethylindolin-2-ylidene)ethylidene)cyclohex-1-enyl)vinyl)-3H-indoliumchloride (Cy7 NHS ester; “CY”);3-(6-(3-azidopropylamino)-6-oxohexyl)-1,1-dimethyl-2-((E)-2-((E)-3-((E)-2-(1,1,3-trimethyl-1H-benzo[e]indol-2(3H)-ylidene)ethylidene)cyclohex-1-enyl)vinyl)-1H-benzo[e]indoliumchloride (Cy7.5 azide);1-(5-carboxypentyl)-3,3-dimethyl-2-((E)-2-((E)-3-((E)-2-(1,3,3-trimethylindolin-2-ylidene)ethylidene)cyclohex-1-enyl)vinyl)-3H-indoliumchloride (Cy7 azide);1,1-dimethyl-3-(6-oxo-6-(prop-2-ynylamino)hexyl)-2-((1E,3E,5E)-5-(1,1,3-trimethyl-1H-benzo[e]indol-2(3H)-ylidene)penta-1,3-dienyl)-1H-benzo[e]indoliumchloride (Cy5.5 alkyne);1-(6-(2-(2,5-dioxo-2,5-dihydro-1H-pyrrol-1-yl)ethylamino)-6-oxohexyl)-3,3-dimethyl-2-((E)-2-((E)-3-((E)-2-(1,3,3-trimethylindolin-2-ylidene)ethylidene)cyclohex-1-enyl)vinyl)-3H-indoliumchloride (Cy7 maleimide);3-(6-(2-(2,5-dioxo-2,5-dihydro-1H-pyrrol-1-yl)ethylamino)-6-oxohexyl)-1,1-dimethyl-2-((E)-2-((E)-3-((E)-2-(1,1,3-trimethyl-1H-benzo[e]indol-2(3H)-ylidene)ethylidene)cyclohex-1-enyl)vinyl)-1H-benzo[e]indoliumchloride (Cy7.5 maleimide);1-(5-carboxypentyl)-3,3-dimethyl-2-((E)-2-((E)-3-((E)-2-(1,3,3-trimethylindolin-2-ylidene)ethylidene)cyclohex-1-enyl)vinyl)-3H-indoliumchloride (Cy7 carboxylic acid);1-Butyl-2-(2(2-[3-[2-(1-butyl-3,3-dimethyl-1,3-dihydro-indol-2-ylidene)-ethylidene]-2-diphenylamino-cyclopent-1-enyl]-vinyl)-3,3-dimethyl-3H-indoliumhexafluorophosphate;1-Butyl-2-(2-[3-[2-(1-butyl-3,3-dimethyl-1,3-dihydro-indol-2-ylidene)-ethylidene]-2-phenyl-cyclopent-1-enyl]-vinyl)-3,3-dimethyl-3H-indoliumhexafluorophosphate;3-Ethyl-2[3-[3-(3-ethyl-3H-benzothiazol-2-ylidene)-propenyl]-5,5-dimethyl-cyclohex-2-enylidenemethyl]-benzothiazol-3-iumiodide;1-Butyl-2-(2-(2-[3-[2-(1-butyl-3,3-dimethyl-1,3-dihydro-indol-2-ylidene)-ethylidene]-2-phenyl-cyclohex-1-enyl]-vinyl)-3,3-dimethyl-3H-indoliumhexafluorophosphate;1-Butyl-2-(2-(2-[3-[2-(1-butyl-3,3-dimethyl-1,3-dihydro-indol-2-ylidene)-ethylidene]-2-phenyl-cyclohex-1-enyl]-vinyl)-3,3-dimethyl-3H-indoliumhexafluorophosphate;3-Ethyl-2-[7-(3-ethyl-3H-benzothiazol-2-ylidene)-hepta-1,3,5-trienyl]-benzothiazoliumiodide;1-Butyl-2-[7-(1-butyl-3,3-dimethyl-1,3-dihydro-indol-2-ylidene)-hepta-1,3,5-trienyl]-3,3-dimethyl-3H-indoliumhexafluorophosphate;2-[3-Chloro-5-[1,1-dimethyl-3-(3-methyl-butyl)-1,3-dihydro-benzo[e]indol-2-ylidene]-penta-1,3-dienyl]-1,1-dimethyl-3-(3-methyl-butyl)-1H-benzo[e]indoliumhexafluorophosphate;2-[5-[1,1-Dimethyl-3-(3-methyl-butyl)-1,3-dihydro-benzo[e]indol-2-ylidene]-penta-1,3-dienyl]-1,1-dimethyl-3-(3-methyl-butyl)-1H-benzo[e]indoliumhexafluorophosphate;3,3-Dimethyl-2-[2-[2-chloro-3-[2-[1,3-dihydro-3,3-dimethyl-5-sulfo-1-(4-sulfobutyl)-2H-indol-2-ylidene]-ethylidene]-1-cyclohexen-1-yl]-ethenyl]-5-sulfo-1-(4-sulfobutyl)-3H-indoliumhydroxide, innersalt, trisodium salt;2-[2-(3-[2-[3,3-Dimethyl-5-sulfo-1-(4-sulfobutyl)-1,3-dihydro-indol-2-ylidene]-ethylidene]-2-phenyl-cyclohex-1-enyl)-vinyl]-3,3-dimethyl-5-sulfo-1-(4-sulfobutyl)-3H-indoliumhydroxide, inner salt, trisodium salt;2-[5-[1,1-Dimethyl-3-(4-sulfobutyl)-1,3-dihydro-benzo[e]indol-2-ylidene]-penta-1,3-dienyl]-1,1-dimethyl-3-(4-sulfobutyl)-1H-benzo[e]indoliumhydroxide, inner salt, sodium salt;2-(8-Hydroxy-1,1,7,7-tetramethyl-1,2,3,5,6,7-hexahydropyrido[3,2,1-ij]quinolin-9-yl)-4-(8-hydroxy-1,1,7,7-tetramethyl-2,3,6,7-tetrahydro-1H-pyrido[3,2,1-ij]quinolinium-9(5H)-ylidene)-3-oxocyclobut-1-enolate(squarylium); and combinations and mixtures thereof. FIGS. 6A-6J showabsorbance and emission spectra for DTCI, HITC, IR144, IR170, AlNc,SiNc, IR792, IR140, ZnNc, and CY respectively. All of the spectra haveemission and absorption peaks at a wavelength longer than about 650 nm.

As mentioned above, individual luminophore performance can be improvedthrough combinations of chemical, purification, architecture, host-guestinteractions and photon management with transparent NIR mirror design,and fabrication. Also, when thiacarbocyanine is the luminophore, thesubstituent alkene length can be varied to alter the emission range.Alternatively, the anions can be modified to improve the emission range.By these modifications, the quantum efficiency can be improved to about50% or greater. In various embodiments, the quantum yield is greaterthan about 10-25%. Preferably, the quantum yield is greater than about60%, or greater than about 70%, or greater than about 80%. Mostpreferably, the quantum yield is greater than about 90%.

A luminophore host is beneficial for: (1) physically separatingmolecules to increase quantum yields, (2) interacting directly andelectronically with the molecules to increase or decrease quantum yieldsthrough polar interactions, and (3) encapsulating the chromophore to actas a barrier to air, moisture, and increase longevity. A PMMA derivativecan, for example, enhance the quantum yield of several molecules fromsolution over a factor of two while allowing for negligible quantumyield reduction over three months in air. Moreover, luminophores can bereadily and directly anchored to amine polymers such aspoly(4-vinylpyridine), and polymers with pendent ammonium salt chains,synthesized from amine polymers or through direct polymerization.

This system results in a new energy pathway to renewable, low-carbonsolar-energy deployment that can overcome many of the social andeconomic challenges associated with traditional PV technologies whileimproving building efficiency. This is achieved with domesticallyabundant materials including carbon based molecules, and nanoclusterscomposed of Br, Cl, C, Mo, N, O, Si, W, phthalic, and naphthalic acid,and thiatricarbocyanine derivatives (i.e., notable industrial metals)and no rare earth, radioactive, or precious metals (e.g., Pt, Ir, etc.).

The efficiency of a luminescent solar concentrator can be defined as:

$\eta_{LSC} = {\eta_{PV}^{*} \cdot \left( {1 - R} \right) \cdot \eta_{abs} \cdot \frac{\eta_{PL}{\eta_{trap}\left( {1 - \eta_{RA}} \right)}}{1 - {\eta_{RA}\eta_{PL}\eta_{trap}}}}$

where R is the front face reflection, η_(abs) is the solar spectrumabsorption efficiency of the luminophore, η_(PL) is the luminescenceefficiency of the luminophore, η_(trap) is the waveguiding efficiency ofthe light, and η_(RA) is the probability of reabsorption. The lighttrapping efficiency is η_(trap)=√{square root over (1−1/n_(wav) ²)}. ThePV efficiency (reported for AM1.5G) normalized by the solar spectrumabsorption efficiency and the quantum efficiency at the luminophorewavelength is:

$\eta_{PV}^{*} = {\left( \frac{\eta_{PV}\left( {{AM}1.5G} \right)}{\eta_{A}\left( {{AM}1.5G} \right)} \right) \cdot \frac{\int{{\eta_{EQE}(\lambda)}{{PL}(\lambda)}d\lambda}}{\int{{{PL}(\lambda)}d\lambda}}}$

where η_(EQE) is the external quantum efficiency, PL is the luminophoreemission spectrum. The thermodynamic limiting η_(PV) only accounts forVoc and FF losses. Because of the light dependence of η_(PV), thiscorrection will become dependent of the geometrical gain of thecollector. For simplicity, it is assumed that the waveguided light fluxremains close to 1-sun.

Reabsorption and forward emission losses can be estimated by:

$\eta_{RA} = \frac{\begin{matrix}{{\int}_{0}^{\infty}d\lambda{\int}_{\theta_{crit}}^{\pi/2}d\theta{\int}_{{- \pi}/4}^{{- \pi}/4}\sin(\theta){{PL}(\lambda)}} \\{\left( {1 - {\exp\left\lbrack {{- \varepsilon}(\lambda)C\frac{Lt}{2t_{0}{\sin(\theta)}{\cos(\phi)}}} \right\rbrack}} \right)d\phi}\end{matrix}}{{\int}_{0}^{\infty}d\lambda{\int}_{\theta_{crit}}^{\pi/2}d\theta{\int}_{{- \pi}/4}^{{- \pi}/4}{\sin(\theta)}{{PL}(\lambda)}d\phi}$

where the critical angle (emission cone) is θ_(crit)=sin⁻¹(1/n_(wave)),ϵ is the molar absorptivity, C is the concentration, L is the platelength, θ is azimuth relative to the normal of the LSC plane, and ϕ isthe normal rotation coordinate. At moderate phosphor loading, the UV LSCcan retain efficiencies beyond 10 m, which is larger than most typicalwindows. In contrast for the NIR emitters, the efficiency begins to“roll-off” at only 1-10 cm, defining the ideal embedded solar cellspacing.

With the present NLSCs, the short Stokes shift of the NIR harvesters isovercome through the solar cell implantation throughout a waveguidelayer, concomitantly reducing overall optical losses, increasing solarabsorption efficiency, and also increasing the cost. Moreover, thecombination of NIR LSC luminophores with segmented PVs enhances theefficiency output over three fold for a range of high-levels oftransparency over segmented PVs alone. Nonetheless, NIR designs areutilized as a platform for applications requiring high-efficiency, andhigh-CRI. Ultimately, these cells will be implemented with embeddedsolar cell “meshes” or thin-wafers allowing for the combination of highperformance, and ideal aesthetic quality.

The energy harvesting devices are transparent or substantiallytransparent to a human viewer. The color rendering index (CRI) for thedevices can be about 80 or higher. For example, the CRI can be about 80,about 85, about 90, about 95, about 99, or about 100. The averagevisible transmittance of the devices is greater than about 60%. Invarious embodiments, the average visible transmittance is greater thanabout 70%, greater than about 75%, greater than about 80%, greater thanabout 85%, or greater than about 90%. The efficiency of the energyharvesting devices can be greater than about 0.25%, greater than about0.5%, greater than about 0.75%, greater than about 1.0%, greater thanabout 5%, or greater than about 10%. In various embodiments, theefficiency is from about 0.25% to about 10%. Therefore, the efficiencycan be about 0.25%, about 0.5%, about 0.75%, about 1.0%, about 2.0%,about 3.0%, about 4.0%, about 5.0%, about 6.0%, about 7.0%, about 8.0%,about 9.0%, or about 10.0%. In some circumstances, the efficiency may bedecreased in order to provide for a more transparent device. Therefore,in some embodiments, the efficiency may be aesthetically limited.

The present technology includes methods for preparing transparent energyharvesting devices. A solution containing a waveguide redirectingmaterial, such as a scattering nanoparticle, a luminophore or acombination thereof, is mixed with a substrate solution to form aworking solution. The substrate solution comprises (poly)-ethylmethacrylate (PEMA), an acetonitrile solution, (poly)-methylmethacrylate (PMMA) (poly)butyl methacrylate-co-methyl methacrylate(PBMMA), or combinations thereof, wherein the substrate solution is in asolvent. In a preferred embodiment, the substrate solution comprisesPEMA, acetonitrile and PMMA in a ratio of 1:2:1. The working solutioncan be drop cast in a mold. Optionally, one or more photovoltaic solarcells can be placed within the working solution or on a surface of theworking solution. The working solution is dried for several hours underflowing nitrogen to form a solid matrix. If the one or more photovoltaicsolar cells were not placed within the working solution, one such solarcell can be attached to a surface of the matrix with nearly-indexedmatched epoxy. Reflective coatings (mirrors) can be incorporated to thedevice by alternating layer combinations of TiO₂, SiO₂, and Al₂O₃ tooptimize overall color impact and performance.

With this technology, panel characteristics are quickly adapted throughlarge input-variable space (e.g., dye concentration, waveguidethickness, mirror integration, window dimension, etc.) around any ofthese output parameters. As a result, product performance can betailored or customized to achieve the necessary combination of opticalperformance, power production and installed cost to match a variety ofspecifications for building designers looking to incorporate energyharvesting into the building skin. Additionally, this system can beseamlessly integrated into the building's electrical system. Thegenerated electricity can then be stored locally and used asdirect-current power or inverted to AC to supplement the building grid,capable of supplying a sizable portion of a building's perimeter zoneelectricity needs at the point of utilization.

As described above, the present technology provides for a lightharvesting system that can be integrated into a building's window glass.However, the light harvesting system can also be layered over siding.Because the light harvesting system is transparent, it cannot be seen bya human viewer looking at the siding, whether the siding is on a house,vehicle, or a device. The light harvesting system can also be integratedinto displays on television monitors, computer monitors, mobile devices,such as cell phones and media players, and signs, such as electronicbillboards. The transparent nature of the light harvesting system allowsit to be integrated into almost any device, vehicle, or building, thatcan benefit from harvesting power from the sun.

Embodiments of the present technology are further illustrated throughthe following non-limiting example.

EXAMPLE NIR Harvesting TLSC

Experimental

Organic salt solution preparation:2-[7-(1,3-dihydro-1,3,3-trimethyl-2H-indol-2-ylidene)-1,3,5-heptatrienyl]-1,3,3-trimethyl-3H-indoliumiodide (HITCI) (Exciton) and1-(6-(2,5-dioxopyrrolidin-1-yloxy)-6-oxohexyl)-3,3-dimethyl-2-((E)-2-((E)-3-((E)-2-(1,3,3trimethylindolin-2-ylidene)ethylidene)cyclohex-1-enyl)vinyl)-3H-indoliumchloride (CY) were characterized as received without furtherpurification. Solutions for optical characterization were prepared bydirectly dissolving each compound in dichloromethane at variousconcentrations up to 4 mg/ml.

Optical characterization: Specular transmittance of both solutions andfilms was measured using a dual-beam Lambda 800 UV/VIS spectrometer inthe transmission mode without the use of a reference sample. Emissionspectra and quantum yields for various dyes were measured using a PTIQuantaMaster 40 spectrofluorometer with excitation at 730 nm (HITCI) and700 nm (CY); quantum yield measurements were made using a calibratedintegrating sphere attachment.

Module fabrication: A 5 mg/ml CY dichloromethane solution was mixed with(poly)-butyl methacrylate-co-methyl methacrylate (PBMMA) (Sigma-Aldrich)at a volume ratio of 1:1, yielding a target dye concentration (2 mg/mL)in the polymer composite film. To make luminescent solar cells (LSCs),this mixture was drop cast on 2 cm×2 cm×0.1 cm (for efficiencymeasurements) or 7 cm×7 cm×0.1 cm (for EQE measurements) square quartzor glass substrates (comprising four edges) and allowed to dry for 2hours for each layer, and repeated 3 times, resulting in a layerthickness of approximately 1 mm. A laser-cut, 2 cm×0.1 cm Si cell (NarecSolar) with an efficiency of 15±1% @ 1 sun was attached with or withoutindexed matched epoxy to an edge of the waveguide. For EQE measurements,the other three edges were taped with black electrical tape to blockedge reflection and simplify the geometric configuration. For powerefficiency measurements, two cells were edge-mounted on orthogonal edgesand connected in parallel. The remaining two edges were covered withenhanced specular reflector (Vikuiti, 3M). A thin border area around theLSC edges was masked to avoid any direct illumination of the LSC. Due tothe illumination area of a solar simulator (67005 Xe arc lamp, Newport),plate lengths for power efficiency measurements were limited to lessthan about 0.05 m, and for EQE measurements, less than about 0.2 m.

Module Testing: Position-dependent EQE measurements were obtained bydirecting a monochromatic excitation beam from a fiber perpendicular tothe LSC at various distances (d) from a single edge-mounted Si PV. Themeasured quantum efficiency was corrected by the factor,g=π/tan⁻¹(L/2d), that accounts for the different angle subtended by thesolar cell at each spot distance, where L is the length of the edge ofthe LSC. The external quantum efficiency (EQE) was measured utilizing aNewport calibrated Si detector. Current density versus voltage (J-V)measurements were performed under simulated AM1.5G solar illuminationcorrected for solar spectrum mismatch.

Optical Modeling: A PV efficiency (reported for AM1.5G) is normalized bya solar spectrum absorption efficiency and a quantum efficiency at theluminophore wavelength. Reabsorption and forward emission losses wereestimated with luminophore emission spectrum, molar absorptivity,concentration, LSC length, LSC system optical efficiency, and EQE werenumerically evaluated in Matlab as a function of plate size, andluminophore concentration at fixed blend thicknesses.

Results

Two promising cyanine derivatives,2-[7-(1,3-dihydro-1,3,3-trimethyl-2H-indol-2-ylidene)-1,3,5-heptatrienyl]-1,3,3-trimethyl-3H-indoliumiodide (HITCI) and1-(6-(2,5-dioxopyrrolidin-1-yloxy)-6-oxohexyl)-3,3-dimethyl-2-((E)-2-((E)-3-((E)-2-(1,3,3-trimethylindolin-2-ylidene)ethylidene)cyclohex-1-enyl)vinyl)-3H-indoliumchloride (CY), were analyzed and the system was utilized to explore theimpact of a Stokes shift. CY and HITCI molecular structures are shown inFIG. 7B. The absorption and emission spectra of the luminophores areshown in FIG. 8A. The absorption spectra peaks are at 742 nm for CY and733 nm for HITCI with little visible absorption, and NIR emission peaksare at 772 and 753 nm for CY and HITCI, respectively. The Stokes shift,defined as the wavelength difference between the absorption and emissionpeaks, is 30 nm for CY and 20 nm for HITCI. The Stokes shift of the twomaterials helps demonstrate the difference in assembled LSC performanceand is also an important parameter to predict large-area scalability.

The luminescence quantum yield (QY) dependence on the concentration wasexplored to understand the photophysical behavior of these luminophores.The measured QY and absorption of CY and HITCI in dichloromethanesolutions are shown in FIG. 8B as a function of dye concentration. TheQY data of FIG. 8B was fit to a model with a concentration dependentnon-radiate rate and the relationships between intrinsic non-radiativerate for isolated clusters k_(nR0), rate for luminescence k_(R), andconcentration quenching scaling constant α were calculated. For CY,k_(nR0)/k_(R)=5.8, α/k_(R)=2.5 ml/mg, while for HITCI,k_(nR0)/k_(R)=2.5, α/k_(R)=2.2 ml/mg. The critical concentrations for CYand HITCI, defined here as the concentration where the QY is half of themaximum, are 2 mg/L and 1 mg/L, respectively. FIG. 7C shows thetransparent LSC waveguide incorporating the CY luminophore. Thespectrally resolved EQE of the overall LSC system of different platesizes are shown in FIG. 3(c), which exhibits a peak at around 760 nm,matching the absorption spectrum and the calculated EQE in FIG. 9C.Shown in FIG. 9B is the current voltage characteristic of the 2 cm×2cm×0.1 cm LSC system for the two edge mounted Si solar PVs (seeExperimental section for details); the measured intensity dependence ofthe assembled CY LSC is shown in FIG. 9A, which is largely dependent ofthe intensity dependence of the edge mounted Si cells. The measuredshort circuit current density (Jsc) of the overall system under 1.0±0.1sun was 1.2±0.1 mAcm⁻², with an open circuit voltage (Voc) of 0.50±0.01V and a fill factor of 0.64±0.02 leading to an efficiency 0.38±0.05% forthe CY luminophore. The calculated short circuit current density fromintegrating the product of the EQE and the AM1.5G solar spectrum, is1.0±0.1 mAcm⁻², which is within error of the measured photocurrents. Thecorresponding average visible transmittance and color rendering indexfor the LSC system is 86%±1% and 94, respectively, compared to 90%±1%and 100 for the glass substrate alone and is slightly better inaesthetic quality to UV-only TLSCs.

FIG. 10 shows area-scaling calculations of optical efficiency of the CYLSC with modeled Stokes shift as a function of LSC length. When theStokes shift (S) is below 30 nm, the critical plate length is around 1-2cm while increasing the S can significantly increase the critical platelength to >1 m for S>100 nm. The information presented in FIG. 10provides information for designing materials to improve scalability.

Discussion

Luminophore Photophysics

The individual non-radiative mode for the luminophores is larger thanthe radiative rates, leading to moderately-low quantum yields. This istrue for many of the demonstrated NIR fluorophores and there continuesto be a significant effort to improve QY in this spectral range for bothmedical applications and light emitting diodes. Here, the quantum yielddecreases with increasing concentration due to excited state(non-radiative) quenching caused by particle-particle interactions athigher concentrations; these interactions persist even into dilutesolutions. Accordingly, the concentrators designed herein utilizethicker layers of dilute concentrations to maintain both high quantumyield and high absorption efficiency, following the Beer-Lambert law.This criterion was utilized when beginning with a design concentrationof 5 mg/L for a high quantum yield, and a molar absorption coefficientof 1.45×10⁸ Lmol⁻¹m⁻¹ for Cy at 760 nm, leading to an optimalthicknesses of 1.0 mm for near complete NIR absorption. Also, areduction in reabsorption losses is realized by utilizing thicker layersof dilute concentrations.

LSC Design

The efficiency of the transparent LSCs is governed by: solar spectrumabsorption efficiency, luminophore photoluminescence efficiencywaveguide (trapping) efficiency, transport (re-absorption) efficiency,and solar cell efficiency. The optical efficiency comprises waveguidingefficiency, transport efficiency, and luminescence efficiencies.Luminophore photoluminescence efficiency could be improved by employinghigher quantum yield dyes or optimizing dye-polymer interactions toachieve higher quantum yields. Solar spectrum absorption efficiency canbe improved by increasing dye concentration or by increasing blendsthickness. However, there is a trade-off between quantum yield andconcentration. Solar cell efficiency could be enhanced by utilizingsolar cells with higher efficiency. Due to the monochromatic emissionnature of the LSC, only single junction PVs can be attached around eachindividual LSC, which limits the overall system efficiency to less thanthe PV efficiency directly. It is also possible to stack complimentarytransparent LSCs with different/absorption ranges on top of each other,whereby each LSC is individually coupled to their ideal PV. If lessexpensive higher bandgap cells (GaAs, and GaInP) become available, itcould boost the efficiency for η_(LSC) over 30% compared to Si,particularly for the LSCs demonstrated herein. However, for cost andavailability considerations, lower-cost Si PVs with AM1.5G with a solarefficiency of 14-16% were utilized for the proof-of-principledemonstration described herein.

Reabsorption losses limit the performance of the LSCs fabricated in thiswork, due to the moderately low stokes shift for the dye. Indeed, it haslong been recognized that a LSC's performance is often limited byreabsorption losses, particularly for dyes with modest S and largerplate sizes. The calculated optical efficiency in FIG. 10 shows that anincrease of Stokes shift from 30 nm to 80 nm can improve the criticallength, defined herein as the LSC length where the optical efficiency ishalf of the maximum, from 3 cm to 30 cm, where an LSC size of 30 cm isappropriate for many LSC applications. To fully maximize the large-areascaling of these devices, the design of high quantum-yield moleculeswith larger Stokes shift is favorable. However, there is a balance inconsidering the ideal Stoke shift. PVs are limited to those that havehigh quantum efficiency at the luminophore emission peak, but one alsomust consider maximizing the bandgap to minimize PV voltage losses. WithSi PVs, the maximum Stoke shift is limited to less than about 200 nmwith the expectation of harvesting a 200-300 nm slice of the NIRspectrum. For GaAs this maximum S is even more restricted.

One approach to obtain better scalability is by improving the quantumyield to closer to 100% through optimization of luminophore-hostinteractions and molecular design. For materials with QY close to 100%,each reabsorption event leads to another remission event, therebyreducing transport losses. However, it should be noted that eachabsorption/remission event appears as additional scattering becauseradiative emission is typically isotropic, and therefore wouldeventually lead to greater front/back-side losses for larger platelengths. Another important approach to improve the scalability for lowStokes shift materials is to embed highly-segmented solar cellmicro-arrays as meshes throughout the LSC waveguide to essentiallycreate a series of “micro-LSCs,” allowing for minimal reabsorptionlosses and additional contributions from the segmented PV. Consideringthese approaches, and by combining these LSCs with UV-TLSCs,efficiencies of greater than about 1% are readily achievable, andefficiencies approaching 10% are possible.

Conclusion

In conclusion, novel transparent luminescent solar concentrator devicescomprising fluorescent organic salts, which selectively harvest NIRphotons, have been designed and fabricated. The firstvisibly-transparent NIR-harvesting TLSC with a non-tinted transparencyof 86%±1% in the visible spectrum combined with an efficiency of0.4%±0.03% and potential for efficiencies up to 10% due to the largefraction of photon flux in the near-infrared has been demonstratedherein. The experiments and modeling show that the development of largerStokes shift near-infrared luminophores, optimization ofluminophore-host interactions, and fabrication of embedded segmented-PVconfigurations could reduce reabsorption losses and increase systemefficiency over large areas. These transparent NIR LSCs provide anentirely new route to transparent light-harvesting systems withtremendous potential for high defect tolerances and processability.

What is claimed is:
 1. An energy harvesting device comprising: awaveguide including, a substrate, and a waveguide redirecting materialin contact with the substrate, the waveguide redirecting material beinga cyanine or salt thereof, a squarylium, a carbon nanotube, athiacarbocyanine or salt thereof, a naphthalocyanine or derivativethereof, a phthalocyanine or derivative thereof, a phorphyrin, or acombination thereof; and a photovoltaic cell coupled to the waveguide,wherein the energy harvesting device is visibly transparent, having anaverage visible transmittance (AVT) of sunlight through the device ofgreater than about 50% and a color rendering index (CRI) of sunlightthrough the device of greater than about 80 referenced to the AM1.5Gspectrum at normal incidence to the waveguide.
 2. The energy harvestingdevice according to claim 1, wherein the AVT is greater than or equal toabout 60%.
 3. The energy harvesting device according to claim 1, whereinthe waveguide redirecting material is a cyanine or salt thereof, asquarylium, a carbon nanotube, a naphthalocyanine or derivative thereof,a phthalocyanine or derivative thereof, a phorphyrin, or a combinationthereof, and the AVT is greater than or equal to about 70%.
 4. Theenergy harvesting device according to claim 1, wherein the waveguideredirecting material is a cyanine or salt thereof, a squarylium, anaphthalocyanine or derivative thereof, a phthalocyanine or derivativethereof, or a combination thereof, and the AVT is greater than or equalto about 80%.
 5. The energy harvesting device according to claim 1,wherein the AVT is less than 90%.
 6. The energy harvesting deviceaccording to claim 1, wherein the waveguide redirecting material is acarbon nanotube, a thiacarbocyanine or salt thereof, a naphthalocyanineor derivative thereof, a phthalocyanine or derivative thereof, aphorphyrin, or a combination thereof the CRI is greater than or equal toabout
 85. 7. The energy harvesting device according to claim 1, whereinthe waveguide redirecting material is a carbon nanotube, anaphthalocyanine or derivative thereof, a phthalocyanine or derivativethereof, or a combination thereof the CRI is greater than or equal toabout
 90. 8. The energy harvesting device according to claim 1, whereinthe waveguide redirecting material has a quantum yield of luminescenceof greater than about 20%.
 9. The energy harvesting device according toclaim 1, wherein a waveguide includes a top surface, a bottom surface,and an edge surface, the waveguide is configured to receive sunlightthrough the top surface, the bottom surface, or both the top surface andthe bottom surface, and the photovoltaic cell is on the edge surface.10. The energy harvesting device according to claim 1, wherein awaveguide includes a top surface, a bottom surface, and an edge surface,the waveguide is configured to receive sunlight through the top surface,the bottom surface, or both the top surface and the bottom surface, andthe photovoltaic cell is embedded within the waveguide, on the topsurface of the waveguide, or on the bottom surface of the waveguide. 11.The energy harvesting device according to claim 1, further comprising: aphotovoltaic array comprising the photovoltaic cell.
 12. The energyharvesting device according to claim 11, wherein the photovoltaic arraycomprises a laser-patterned mesh.
 13. The energy harvesting deviceaccording to claim 11, wherein the photovoltaic array comprises wires,microwires, nanowires, ribbons, slivers, spheres, dots, or acombinations thereof.
 14. The energy harvesting device according toclaim 1, wherein the photovoltaic cell includes a photoactive materialselected from the group consisting of: germanium (Ge); amorphousgermanium (a-Ge); gallium (Ga); gallium arsenide (GaAs); silicon (Si);amorphous silicon (a-Si); silicon-germanium (SiGe); amorphoussilicon-germanium (a-SiGe); gallium indium phosphide (GaInP); copperindium selenide, copper indium sulfide; copper indium gallium selenide;copper indium gallium sulfide; cadmium telluride (CdTe); a perovskite(PV); or a combination thereof.
 15. The energy harvesting deviceaccording to claim 1, wherein the substrate comprises a materialselected from the group consisting of: glass, plastic, poly(methylmethacrylate) (PMMA), poly-(ethylmethacrylate) (PEMA), (poly)-butylmethacrylate-co-methyl methacrylate (PBMMA), or a combination thereof.16. The energy harvesting device according to claim 1, wherein thewaveguide redirecting material is on a surface of the substrate.
 17. Theenergy harvesting device according to claim 1, wherein the waveguideredirecting material is embedded within the substrate.
 18. The energyharvesting device according to claim 1, wherein the waveguideredirecting material has a strongest peak absorbance of light at awavelength of greater than about 650 nm and a strongest peak emission oflight at a wavelength of greater than about 650 nm.
 19. The energyharvesting device according to claim 18, wherein the waveguideredirecting material has no peak absorption in the visible spectrum. 20.The energy harvesting device according to claim 1, wherein the waveguideredirecting material has a strongest peak absorbance of light at awavelength of greater than about 650 nm, and the waveguide redirectingmaterial has a strongest peak emission of light at a wavelength ofgreater than about 650 nm.
 21. The energy harvesting device according toclaim 1, further comprising: a wavelength-selective mirror coupled to asurface of the waveguide, the wavelength-selective mirror beingtransparent to visible light and reflective to NIR light.
 22. The energyharvesting device according to claim 1, wherein the waveguideredirecting material includes a blend of a cyanine or salt thereof and asquarylium.
 23. The energy harvesting device according to claim 1,wherein the waveguide redirecting material includes a thiacarbocyanineor salt thereof, a naphthalocyanine or derivative thereof, aphthalocyanine or derivative thereof, or a combination thereof.
 24. Anenergy harvesting device comprising: a waveguide including, a substrate,and a waveguide redirecting material in contact with the substrate, thewaveguide redirecting material being a cyanine or salt thereof, asquarylium, a carbon nanotube, a thiacarbocyanine or salt thereof, anaphthalocyanine or derivative thereof, a phthalocyanine or derivativethereof, a phorphyrin, or a combination thereof, the waveguideredirecting material configured to create luminescence at a quantumyield of luminescence of greater than about 20%; and a photovoltaic cellcoupled to the waveguide, wherein the energy harvesting device isvisibly transparent, having an average visible transmittance (AVT) ofsunlight through the device of greater than about 50%.
 25. An energyharvesting device comprising: a waveguide including, a substrate, and awaveguide redirecting material in contact with the substrate, thewaveguide redirecting material being a cyanine or salt thereof, asquarylium, a carbon nanotube, a thiacarbocyanine or salt thereof, anaphthalocyanine or derivative thereof, a phthalocyanine or derivativethereof, a phorphyrin, or a combination thereof, the waveguideredirecting material configured to create luminescence at a quantumyield of luminescence of greater than about 20%; and a photovoltaic cellcoupled to the waveguide, wherein the energy harvesting device has acolor rendering index (CRI) of sunlight through the device of greaterthan about 80 referenced to the AM1.5G spectrum at normal incidence tothe waveguide.